Abstract:  To explore and observe the effects of Tumstatin 19 peptide on the proliferation ability of MDA-MB-231 breast cancer cells and on the growth of implanted tumor in the animal models of breast cancer. Methods: Thiazolyl blue tetrazolium bromide ( MTT ) assay was used to detect the inhibitory action of Tumstatin 19 peptide on the growth of MDA-MB-231 breast cancer cells. The orthotopically implanted breast cancer models in nude mice were divided into four groups. The 19 peptide, cyclophosphamide, cyclophosphamide + 19 peptide, and solvent treatment were intraperitoneally given every other day in to the nude mice of each group. The changes after treatment for the implanted tumor in each group were recorded, and the results were statistically analyzed. Results: The results of the MTT assay showed that the viability of the MDA-MB-231 breast cancer cells  decreased with increasing concentrations of the 19 peptide. The side effects were less in the Tumstatin 19 peptide treatment group and combination groups than in the cyclophosphamide group. There were statistically significant differences in tumor weight among the treatment groups. The P values were 0.010 and 0.032, respectively. There was statistical significance in the difference of the tumor weights between the treatment and the control group. The mean value of P was less than 0.05. Conclusion: In vitro, Tumstatin 19 peptide can inhibit the growth of MDA-MB-231 breast cancer cells. In vivo, the effect of Tumstatin19 peptide is gentle with less toxicity. When the 19 peptides combine with cyclophosphamide, the inhibition of tumor growth is enhanced, and the toxicity of cyclophosphamide is significantly reduced.